Electrophotography is a method comprising forming an electrostatic latent image on a photoreceptor; depositing a toner onto the image to form an imagewise pattern; and transferring the toner to an object. Electrophotography includes two major categories: two-component development and one-component development. In the two-component development, a developer contains two components of a carrier and a toner, and a magnetic carrier is often used as a carrier.
In the two-component development with a magnetic carrier, a developer is stirred and mixed in a developing vessel such that a toner is electrostatically charged to a desired extent by friction between the carrier and the toner. The mixed developer is then fed to a magnet roll (hereinafter, referred to a roll), and spikes of the developer are formed along magnetic lines. The spikes are called magnetic brushes. The magnetic brushes are allowed to come into contact with a surface of a photoreceptor, and thereby the charged toner is deposited onto the surface in conformity with the electrostatic latent image to form a desired image.
While the toner is transferred onto the photoreceptor, the magnetic carrier remains on the roll, and is recovered and reused. Hence, the carrier preferably has a high longevity.
Electrophotography is utilized in a wide range of fields including a copying machine, a printer and a facsimile. In these fields, there is a need to improve image quality, resolution, gradation properties, and reproducibility of fine lines. Deterioration of image quality is partially due to a leak of the potential of the electrostatic latent image via the carrier. With the lower electric resistance of the carrier, the leak phenomenon is more likely to occur. However, even for a carrier initially having a high electric resistance, the electric resistance may be reduced by dielectric breakdown when a high voltage is applied. In such a case, the carrier may contribute to a leak.
Recently, a high bias potential is often applied between a photoreceptor and a roll to achieve high image quality. At such a high bias potential, a conventional carrier tends to cause dielectric breakdown. Hence, there is a need for an electrophotographic development carrier having a high dielectric breakdown voltage and a high longevity.
In order to improve image quality, it is necessary to adjust the saturation magnetization of a magnetic carrier into a certain range, as well as to enhance dielectric breakdown voltage. When the saturation magnetization is too small, the Image quality is deteriorated because carriers are scattered and undesirably deposited on an object. When the saturation magnetization is too large, the spike becomes too hard to maintain image quality.
As a conventional ferrite carrier having a high dielectric breakdown voltage, a Cu—Zn-based ferrite (for example, see Japanese Patent No. 1,688,677) and a Mn—Mg-based ferrite (for example, see Japanese Patent No. 3,243,376) have been used. Under recent environmental regulations, however, it Is desired to reduce the amount of heavy metals used such as Cu, Zn, Mn, Co and Ni. For example, under Title 22 of the State Law of California, Ni, Cu, Zn and the like are control subjects. Moreover, under the PRTR system, Mn compounds are designated as compounds that may be harmful to the health of human beings and an ecosystem.
Magnetite (Fe3O4) has been conventionally known as a magnetic carrier in compliance with environmental regulations; however, magnetite has a problem of a low dielectric breakdown voltage. Moreover, magnetite has a low electric resistance. Due to this low electric resistance, when alternating voltage is applied, a leak phenomenon occurs upon development even if insulating properties are improved by coating with various resins. In order to achieve a high electric resistance for magnetite, there has been an attempt to heat a material in air to form a high electric resistance and non-magnetic phase (Fe2O3 phase), which co-exists with magnetite. With the increased percentage of the Fe2O3 phase in the carrier, the dielectric breakdown voltage becomes higher. However, coercive force is disadvantageously increased. The increased coercive force causes agglomeration of carrier particles, resulting in lowered flowability. The lowered flowability raises a new problem that it is difficult to obtain image quality comparable to that for the ferrite carrier. In addition, since magnetite has a relatively high saturation magnetization, the spike of the magnetic brush becomes too hard.
As an oxide carrier capable of being controlled to have a desired saturation magnetization and meeting environmental regulations, Mg—Fe—O based powder and a method of producing the powder are reported (see Japanese Patent No. 2,860,356). According to this method, a binder is added as a reducing agent, and then sintered in an inert gas atmosphere. Therefore, the valence of Fe can be kept low. As a result, various phases such as magnetite phase and MgO phase co-exist in the resulting powder. Hence, there still remains a problem of a low dielectric breakdown voltage derived from magnetite.
A Mg-based ferrite in the form of a single phase of Mg and Fe is obtained by sintering a stoichiometric composition in air. While this Mg-based ferrite has a high dielectric breakdown voltage, it has a low saturation magnetization from 20 to 25 emu/g.
Accordingly, there still remains a need to realize both a proper saturation magnetization and a high dielectric breakdown voltage simultaneously.